This invention relates to data transmission in general and more particularly to an improved method for obtaining a more secure data transmission between a transmitter and one or more receivers.
In various transmission systems for transmission of messages, typically in digital form, between military operating units, for example, the ability to achieve secure transmissions becomes a problem.
In many communication system applications there is a need for security from detection, demodulation and interference or jamming. Techniques have been developed to provide for these security needs, and among them are what is referred to as spread spectrum techniques. These techniques are explained in some detail in the publication, "Spread Spectrum Techniques ", Ed. by R. C. Dixon, IEEE Press, 1976.
Two of the techniques disclosed in the article are pertinent to the subject matter of this invention. The first is the concept of encoding the information to be transmitted so that unauthorized reception yields no useful information, this is generally referred to as a direct sequence modulated system. The encoding is usually accomplished by modulating the incoming digital information with a higher speed code sequence which is then used to suppressed-carrier modulate a Radio Frequency carrier. The high speed code sequence determines the Radio Frequency bandwidth since it dominates the modulating function. The signal is then received in a receiver which multiplies the wide-band signal with a locally generated replica thus collapsing the wide-band signal into a bandwidth resulting in a bandwidth having only the information transmitted. The information is then demodulated.
The other technique is the use of different frequencies during certain time intervals, this is usually referred to as the frequency hopping technique. Present frequency hopping systems utilize a code sequence to select the frequency employed at any one particular time.
In both the direct sequence modulated system and the frequency hopping system it is common to transmit messages in serial pulse format with terminals receiving only one message at a time. Typically, the message is preceded by what is called a sync preamble. The sync preamble is a coded message which permits a receiver to detect a fact that a message is coming and to place it in a position to receive that message.
In the frequency hopping system, a code which can consist of up to 32 what are known as "chips" may be transmitted at each frequency. Thus, for example, the transmitter will first transmit at a first frequency f1 a code, c1 which includes 32 chips. Typically this is done by transmitting a carrier burst for a duration of 6.4 micro seconds. The carrier can be phase modulated so as to present the 32 chips each lasting for 200 nano seconds. Each chip can have one of two phase values, i.e., it can be either in phase or out of phase. After transmitting the first code c1 at the first frequency f1, the transmitter then transmits a second code c2 at a different frequency f2. Next, a third code is transmitted either at a different frequency f3 or possibly, again, at the same frequency f1. For the purposes of discussion assume it is at f1. It then transmits a fourth code c4 at another frequency which can be a separate frequency, again, but which for the sake of the present discussion will be assumed to be at f2.
At the receiver end, these four codes which are transmitted must be detected and decoded. Both the transmitter and receiver are automatically programmed to continually change the codes, and the transmitter and receiver are synchronized. Very accurate synchronization systems are known in the art, for example that disclosed in U.S. Pat. No. 4,005,266. The synchronization system described in the aforementioned patent permits one or more local time base systems to be synchronized to a master base system having an oscillator driven clock.
The time synchronization error between the systems is measured at predetermined sampling times and frequency and phase correction signals for the local oscillators and time correction signals for the local clocks are derived from the measured error at each of the sampling times. The oscillator correction signals are applied to the local oscillator and the time correction signals are applied to the local clock at gains which are a function of the magnitude of the error and the number of sampling times between corrections, so that corrections are made which are based upon the rate-of-change of error over the recent history of prior error corrections and not merely upon the instantaneous value of the measured error at each sampling time.
The apparatus for synchronizing master and local time base systems disclosed in that patent provides rapid, accurate slaving of remotely located local clocks and oscillators to a master clock and oscillator through the use of coded signals. Depending on the amount of security desired the conditions at the receiver may be set up such that reception of any one of the codes is sufficient to put the receiving system in a mode which enables it to receive a message. At the other extreme, the condition that all four codes must be received may be a condition precedent to receiving the message.
The typical manner of constructing the receiving means to respond to a transmission of this type in the prior art was to provide two separate receivers, one operating at the frequency f1 and the other operating at the frequency f2. Associated with each receiver would be one or more correlators for decoding or correlating the transmitted code with the preset reference.
With regard to the codes used, it should be noted that the codes are continually changed for purposes of security. Thus, for any given transmission there will be a series of codes such as c1, c2, c3, and c4. The codes for the next transmission might be c5, c6, c7, and c8. Both the transmitter and receiver are automatically programmed to continually change these codes and are synchronized as explained above so that the receiver knows at a given time which codes the transmitter will be sending. The details of exactly how this is done is beyond the scope of the present invention.
As a code is received by the receiver, it is fed in to the correlator. As noted above, it will be a burst at a carrier frequency which is phased modulated. For example, in phase could be considered to be zero and out of phase to be a one. Thus, a code containing 32 bits of phase modulated information will be received. In the correlator, the received code is compared with the predetermined code, which the receiving station knows should be sent at this time. Only when the same code is received is the message considered proper. Thus, the correlator compares the received 32 chip signal with a reference 32 chip signal and, if they are the same, provides a maximum signal output indicating that the code is proper.
Correlators useful for this purpose are well known. Typically such a correlator comprises an acoustic surface wave delay line in which an acoustic wave is set up in a piece of quartz. Spaced along the quartz are 32 detectors representing the 32 chips. The outputs of the detectors are either provided directly or through an invertor to a summing point with a signal from the summing point indicating the correctness of a code. At each of the 32 positions the signal can be fed directly or inverted. This is controlled in accordance with the reference signal which is predetermined and which is to appear at a given time. Thus, a code sequencer or what is referred in the aforementioned Dixon publication as pseudo random noise generator preprograms the correlators to accept only the proper code.
Spread spectrum systems offer many advantages in addition to the inherent message privacy or security advantage. One of these advantages is interference rejection which occurs as a result of the spectrum spreading and subsequent de-spreading necessary for the operation of the receiver. This type of systems offer an improvement in the signal-to-noise ratio of its receiver's Radio Frequency input and its baseband output. A measure of that improvement is the "process gain", which is the ratio of the spread, or transmitted bandwidth, to the rate of the information sent. The amount of interference that a receiver can withstand while operating a tolerable output signal-to-noise ratio is referred to as the antijamming margin, which is determined by the system's process gain.
In accordance with the prior art arrangement, one thus requires a separate receiver for each frequency. In many systems more than two frequencies are required, thus multiplying the number of receivers and the cost and size of the system. It thus becomes evident that there is a need for an improved manner of carrying out such communications while still maintaining good security and antijamming properties.